Credit initialization in systems with proactive flow control

ABSTRACT

The inventive mechanism initializes the credit and debit registers used in a network computer system that uses proactive flow control. During initialization, the credit register is initialized to zero, while the debit register is initialized to the amount of queue space available in that particular chip release. Once the debit register is non-zero, it eventually releases the credits. These credits will be added to the credit register. These activities of adding and releasing credits take place during normal operation. Thus, the sender and destination nodes do not have to undergo any other initialization stages for setting up the credits.

TECHNICAL FIELD OF THE INVENTION

This application relates in general to network computer systems, and inspecific to mechanism for initializing credit in a queue flow controlsystem.

BACKGROUND OF THE INVENTION

The communication between different nodes in a network computer systemoccurs in a producer-consumer fashion. A sender or producer node sends atransaction to one or more destination or consumer node(s). Eachdestination node stores the transaction in an internal queue and‘consumes’ it at a later point of time. For example, in a multiprocessorsystem, a snoop request by the source node is stored in an internalqueue by all the destination nodes. Each node will act on that snooprequest at some later point in time, and then removes the request fromits queue. Thus, a producer should not send a transaction to adestination node that does not have the space required to store thetransaction in its queue. The space requirement is ensured by theunderlying flow-control mechanism.

The flow-control mechanism may be reactive or proactive. In the reactiveapproach, the destination node tracks its own queues and sends aqueue_full signal to the respective sending nodes when the queues arefilling up. However, the proactive flow-control mechanism is consideredto be more useful in systems with a point-to-point interconnecttopology, for example cross-bar, ring, mesh, or hypercube. In theproactive approach, the sender node keeps track of the amount ofavailable queue space in the destination node through the amount of‘credits’ it has for the corresponding queue in the destination node.Each credit translates to a certain number of entries in the destinationqueue. The number of available credits is maintained in a register inthe sender node. This extra space register is referred to as no_credit.Each node will have one no_credit register for every queue in everydestination node to which the node can send a transaction. A sender doesnot send a transaction if the amount of space required by thetransaction is more than the number of credits it possesses for thequeue in the destination node.

The destination node ‘releases’ these credits to the sender, when it‘consumes’ a transaction from its queue. A destination node may chooseto release credits as it is unloading a transaction. Or the destinationnode may release the credits after the transaction has been unloaded.The amount of credits released by the destination node corresponds tothe amount of space freed up in the queue by consumption of atransaction. The destination node keeps track of the amount of queuespace it will be releasing in a debit register. This register isreferred to as no_debit. The destination node releases the credits fromthe no_debit register when it gets an opportunity to send a transactionto the sender node that had sent the transaction(s) earlier. Note thatthe destination node may append the credits onto another transactionpassing through the sender node.

After releasing credits, the no_debit register is decremented by theamount of credits released. On receipt of these credits, the sender addsthe credits to the amount of credits available, no_credit, for thatdestination node. This way the sender keeps track of the amount of queuespace it can safely use in the destination node for forwardingtransactions.

Note that each node many have more than one queue associated withanother particular node. For example, each node may have a request queueand a response queue to store requests and responses from another node.Therefore, separate credits will be maintained for each queue by thenode.

A problem occurs with the initialization of credits in the no_credit andno_debit registers, either during an initial start up or after a reset.Different components of the system may undergo different designrevisions and the corresponding queue sizes may change over time for thesame component. Thus, the sender nodes cannot be hardwired to assume thequeue size in the destination nodes since that size may change withchanges made to the components of the destination node.

Therefore, during power up, the sender does not know how many credits toallocate in the registers for each queue in the destination node. Theprior art uses a software mechanism with additional hardware to performa credit initialization. Both the credit and debit registers are set tozero. The software is used to read the maximum number of credits foreach destination node, and then write that into the sender node.However, in order to perform this credit initialization transactioncredits must be available. Thus, this forms a paradox where a creditinitialization cannot take place until credits are available. To solvethis problem, the prior art uses a ‘power on mode’ which allows alltransactions to be sent without using credits. This mode requires extralogic and imposes a certain minimum queue sizing to allow for thereading and writing of registers. Thus, the prior art mechanism requiresthe user to have extensive knowledge of the system.

Therefore, there is a need in the art for a mechanism that allows forcredit initialization without involving software intervention and usingadditional hardware.

SUMMARY OF THE INVENTION

These and other objects, features and technical advantages are achievedby a system and method which employs the mechanisms already in place andused for normal operations. During normal operations, if the sender hascredits, then is may send transactions to the destination node. If thedestination node has debits, then it releases them back to the sendernode. During initialization, the credit register, no_credit, is loadedwith a zero. Thus, the sender node begins operations with no credits,and cannot send any transactions. The debit register, no_debit, isloaded not with zero, but with the maximum credits representing the sizeof its queue. Thus, the destination register begins operations with thedebit register full, and through normal operations, releases the creditsback to the sender node. Thus, the sender node will then have creditsand can then send transactions, and the destination node will have anempty debit register. This results in faster boot times, since theinitialization is completed as soon as the chips are out of their resetstate, which is 1 cycle after reset is withdrawn, because no time isspent on software operations in a start-up mode.

It is a technical advantage of the invention to perform initializationof the queue registers through normal flow control operations.

It is another technical advantage of the invention to eliminate the needfor software credit initialization.

It is a further technical advantage of the invention to eliminate theneed for additional hardware and registers for a power on mode.

It is a further technical advantage of the invention to have faster boottimes, since the initialization is completed as soon as the chips areout of their reset state or 1 cycle after reset is withdrawn.

It is a further technical advantage of the invention that the inventivemechanism will work with any sized queue and does not have any minimumsizing requirements for initialization.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand the specific embodiment disclosed may be readily utilized as a basisfor modifying or designing other structures for carrying out the samepurposes of the present invention. It should also be realized by thoseskilled in the art that such equivalent constructions do not depart fromthe spirit and scope of the invention as set forth in the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 depicts a network system having two nodes interconnected by abus;

FIG. 2 depicts the flow control mechanism used by the sending node intransmitting a packet to the destination node;

FIG. 3 depicts the flow control mechanism used by the sending node inreceiving a packet from another node;

FIG. 4 depicts the flow control mechanism 40 used by the destinationnode in receiving a packet from the sending node; and

FIG. 5 depicts the flow control mechanism in the destination node duringa power on or reset condition of the system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 depicts a simple network system 10, having two nodes, node A 11and node B 12 connected by bus 13. Each of the nodes will have at leasttwo queues, a request queue and a response queue, although each nodecould have more queues. By having separate request and response queues adeadlock situation is avoided. Node A 11 uses its request queue forholding requests that are being sent to node B 12. Node B 12 uses itsrequest queue for holding requests receiving from node A 11. Node A 11uses its response queue for holding response that are being sent to nodeB 12. Node B 12 uses its response queue for holding responses receivingfrom node A 11. Note that for node A 11 to send a response, node A musthave received a request from node B 12, however for purposes ofsimplicity, assume that node A 11 is the sending node, and node B 12 isthe destination node, although either node could be sending orreceiving.

FIG. 2 depicts the flow control mechanism 20 used by node A 11 insending a packet P to destination node B 12. The sending node generatesthe packet P 21 for a particular queue, queue i, which could be theresponse queue or the request queue, or another type of queue. Note thatthe packet P may have been generated elsewhere, and is merely passingthrough node A onto node B. The arbitration and forwarding logic 22 ofnode A determines how much queue space this particular packet P willconsume in queue i of node B. This space is called space_reqd_P. Thelogic then determines if the credit register for queue i of node B,no_credit_B_Qi, has a sufficient number of credits for the spacerequired for packet P. The logic determines 23 whether no_credit_B_Qi isgreater than or equal to space_reqd_P. If not, then node A 12 waits forcredits 24 to be released from node B 12 before sending packet P. Ifthere is enough credits, then packet P is scheduled 25 for delivery tonode B 12. The packet P could go to a dispatch queue or be sent out tonode B 12. The credit register is decremented 26 by the amount of spacefor packet P, no_credit_B_Qi is to no_credit_B_Qi minus space_reqd_P.Packet P is then sent out 27 to node B 12.

FIG. 3 depicts the flow control mechanism 30 used by node A 11 inreceiving a packet Q from another node 31. The arbitration andforwarding logic of node A determines 32 whether packet Q has anyreleased credits for node A 11. If packet Q contains credits, then thelogic increments the appropriate registers by the number of credits 33.For example, if packet Q was sent by node B, then the packet may containcredits for credit register no_credit_B_Qi, in which case,no_credit_B_Qi is incremented by the number of credits in the packet.After incrementing or if there were no credits for node A, the logicthen determines whether packet Q is destined 34 for node A 11. If not,the packet is forwarded to the appropriate node 35, which may be eitherthe destination node or the next node in the chain of nodes to thedestination node. If node A is the destination node, then the packet isforwarded 36 to the processing unit inside node A. Note that thissequence assumes that every node is only keeping track of the credits ofthe adjacent nodes. Further note that even if the packet is not destinedfor node A, an adjacent node may have appended credits into the packetfor node A. If credits were passed back to non-adjacent nodes on asystem where there is no store and forward, the credits would bereleased at the destination nodes only.

FIG. 4 depicts the flow control mechanism 40 used by node B 12 inreceiving a packet P 41 via bus C 13 from node A 11, as shown in box 27.Since node B is also a sending node, its arbitration and forwardinglogic determines 42 whether packet P has any released credits for node B12. If packet P contains credits, then the logic increments theappropriate counters by the number of credits 43. After or duringincrementing or if there were no credits for node B, then the logic thendetermines whether packet P is destined 44 for node B 12. If not, thepacket is forwarded to the appropriate node 45, which may be either thedestination node or the next node in the chain to the destination node.If node B is the destination node, then the packet is forwarded 46 tothe processing unit inside node B. Packet P then enters 47 queue i innode B 12. If packet P is a request packet, then queue i is the requestqueue. If packet P is a response packet, then queue i is the responsequeue. After a time period, packet P will leave queue i and be consumed48. For example, if packet P is a request, then as soon as the responseis generated the request is removed. Another example is that the packetmay be moved to another queue within node B. After the packet isconsumed, then credits are released to the debit counter, debit_A_queuei, which is incremented 49 by the appropriate value of the credits. NodeB will unload the credits in the debit register to node A whenappropriate 54. Note that a specific packet, a credit packet, could begenerated by node B to send the credits back to node A, or node B couldappend the credits onto a data or information packet that is eitherdestined node A or passing through node A. Note that node A does nothave to be the destination of the information packet. Further note thatnode B may not have constructed the information packet, node B may justbe passing the packet along the node chain. The entire contents of thedebit register may be loaded into the packet bound for node A, or only aportion of the contents may be loaded. Thus, either several packets or asingle packet may be used to send the credit back to node A. The debitregister, debit_A_queue i is decremented according to the number ofcredits unloaded. Note that a specific wire may be also used to send thecredits back instead of packets. The returned credits will be handled bynode A as shown in FIG. 3.

FIG. 5 depicts the flow control mechanism 50 in node B during a power onor reset condition of the system 10. The arbitration and forwardinglogic of node B first determines whether the reset condition has beenlifted 51. If not, node B waits until the reset condition is lifted.After the reset condition has been lifted, the debit register,debit_A_queue is loaded 52 with the maximum value that queue i canstore. The debit register is then unloaded and decremented using thenormal release mechanisms 53, as shown in FIG. 4. Note that the maximumvalue of queue i is set in hardware, e.g. via a register, since thevalues of the queues of are known at the time of design. Thus, nosoftware is needed.

The arbitration and forwarding logic of node A would also determinewhether the reset condition has been lifted, and if not, node A waitsuntil the reset condition is lifted. After the reset condition has beenlifted, the credit register, no_credit_B_Qi is loaded with the valuezero. A packet will come from node B containing the released credits,which will be handled by node A as shown in FIG. 3. Thus, the creditregister of node A will be properly set to the number of credits thatqueue i in node B can hold.

Note that this mechanism will operate for each queue in node B that isused by node A. Further note that this mechanism will operate for eachqueue in node A that is used by node B, as each node can both send andreceive, and thus will have both credits and debits for the differentregisters.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims.

What is claimed is:
 1. A method for initializing credit in a credit register used for flow control that is resident on a first node of a multinode computer system, the method comprising the steps of loading an initial credit value into a debit register resident on a second node of the multinode computer system; loading the credit register with a zero prior to the step of transferring; and transferring the initial credit value into the credit register by using operational mechanisms of the system.
 2. The method of claim 1, wherein: the initial credit value is equal to a size of a queue that is resident on the second node and receives information from the first node.
 3. The method of claim 1, wherein: the step of loading is performed after a reset condition has been lifted.
 4. The method of claim 1, wherein the step of transferring comprises the steps of: placing the initial credit value into a packet which is bound for the first node; decrementing the debit register by the initial credit value; and sending the packet to the first node.
 5. The method of claim 4, wherein the step of transferring further comprises the steps of: receiving, by the first node, the packet from the second node; determining whether the packet is destined for the first node; forwarding the packet to another node, if the packet is not destined for the first node; and processing the packet, if the packet is destined for the first node.
 6. The method of claim 4, wherein the step of transferring further comprises the steps of: receiving, by the first node, the packet from the second node; inspecting the packet, by the first node, to determine whether the packet includes credits for the first node; and incrementing the credit register by a value of the credits determined to be in the packet.
 7. The method of claim 6, further comprising the step of: sending a subsequent packet to the second node from the first node.
 8. The method of claim 7, wherein the second node includes a queue that receives information from the first node, wherein the step of sending the subsequent packet comprises the steps of: determining an amount of space that the subsequent packet will require in the queue of the second node; comparing the amount of space with a number of credits in the credit register; sending the subsequent packet to the second node if the amount of space is less than the number of credits; and holding the subsequent packet until the amount of space is less than the number of credits if the amount of space is not less than the number of credits.
 9. The method of claim 8, wherein the step of sending the subsequent packet further comprises the step of: decrementing the credit register by the amount of space.
 10. The method of claim 8, further comprising the steps of: receiving, by the second node, the subsequent packet from the first node; inspecting the packet, by the second node, to determine whether the subsequent packet includes credits for the second node; and incrementing a credit register of the second node by the value of the credits determined to be in the subsequent packet.
 11. The method of claim 8, further comprising the steps of: receiving, by the second node, the subsequent packet from the first node; determining whether the subsequent packet is destined for the second node; forwarding the subsequent packet to another node, if the subsequent packet is not destined for the second node; and processing the subsequent packet, if the subsequent packet is destined for the second node.
 12. The method of claim 11, further comprising the steps of: consuming, by the second node, the subsequent packet; and releasing, by the second node, the amount of credits to the first node.
 13. A method for initializing credit in a credit register used for flow control that is resident on a first node of a multinode computer system, the method comprising the steps of: loading an initial credit value into a debit register resident on a second node of the multinode computer system; loading the credit register with a zero prior to the step of transferring; and transferring the initial credit value into the credit register by using operational mechanisms of the system; wherein the initial credit value is equal to a size of a queue that is resident on the second node and receives information from the first node, and the steps of loading are performed after a reset condition has been lifted.
 14. The method of claim 13, wherein the step of transferring comprises the steps of: placing the initial credit value into a packet which is bound for the first node; decrementing the debit register by the initial credit value; and sending the packet to the first node.
 15. The method of claim 14, wherein the step of transferring further comprises the steps of: receiving, by the first node, the packet from the second node; inspecting the packet, by the first node, to determine whether the packet includes credits for the first node; and incrementing the credit register by a value of the credits determined to be in the packet.
 16. A mechanism for initializing credit in a credit register used for flow control that is resident on a first node of a multinode computer system, the mechanism comprising: means for loading an initial credit value into a debit register resident on a second node of the multinode computer system; means for loading the credit register with a zero which operates prior to the means for transferring; and means for transferring the initial credit value into the credit register using operational mechanisms of the system.
 17. The mechanism of claim 16, wherein: the initial credit value is equal to a size of a queue that is resident on the second node and receives information from the first node.
 18. The mechanism of claim 16, wherein: the means for loading operates after a reset condition has been lifted.
 19. The mechanism of claim 16, wherein the means for transferring comprises: means for placing the initial credit value into a packet which is bound for the first node; means for decrementing the debit register by the initial credit value; and means for sending the packet to the first node.
 20. The mechanism of claim 19, wherein the means for transferring further comprises: means for receiving the packet from the second node; means for determining whether the packet is destined for the first node; means for forwarding the packet to another node, if the packet is not destined for the first node; and means for processing the packet, if the packet is destined for the first node.
 21. The mechanism of claim 19, wherein the means for transferring further comprises: means for receiving the packet from the second node; means for inspecting the packet to determine whether the packet includes credits for the first node; and means for incrementing the credit register by a value of the credits determined to be in the packet.
 22. The mechanism of claim 21, further comprising: means for sending a subsequent packet to the second node from the first node.
 23. The mechanism of claim 22, wherein the second node includes a queue that receives information from the first node, wherein the means for sending the subsequent packet comprises: means for determining an amount of space that the subsequent packet will require in the queue of the second node; means for comparing the amount of space with a number of credits in the credit register; means for sending the subsequent packet to the second node if the amount of space is less than the number of credits; and means for holding the subsequent packet until the amount of space is less than the number of credits if the amount of space is not less than the number of credits.
 24. The mechanism of claim 23, wherein the means for sending the subsequent packet further comprises: means for decrementing the credit register by the amount of space.
 25. The mechanism of claim 23, further comprising: means for receiving the subsequent packet from the first node; means for inspecting the packet to determine whether the subsequent packet includes credits for the second node; and means for incrementing a credit register of the second node by the value of the credits determined to be in the subsequent packet.
 26. The mechanism of claim 23, further comprising: means for receiving the subsequent packet from the first node; means for determining whether the subsequent packet is destined for the second node; means for forwarding the subsequent packet to another node, if the subsequent packet is not destined for the second node; and means for processing the subsequent packet, if the subsequent packet is destined for the second node.
 27. The mechanism of claim 26, further comprising: means for consuming, by the second node, the subsequent packet; and means for releasing, by the second node, an amount of credits to the first node, wherein the amount of credits equals the amount of space.
 28. A mechanism for initializing credit in a credit register used for flow control that is resident on a first node of a multinode computer system, the mechanism comprising: first means for loading an initial credit value into a debit register resident on a second node of the multinode computer system; second means for loading the credit register with a zero; and means for transferring the initial credit value into the credit register by using operational mechanisms of the system; wherein the initial credit value is equal to a size of a queue that is resident on the second node and receives information from the first node, and the first and second means for loading operate after a reset condition has been lifted.
 29. The mechanism of claim 28, wherein the means for transferring comprises: means for placing the initial credit value into a packet which is bound for the first node; means for decrementing the debit register by the initial credit value; and means for sending the packet to the first node.
 30. The mechanism of claim 20, wherein the means for transferring further comprises: means for receiving the packet from the second node; means for inspecting the packet to determine whether the packet includes credits for the first node; and means for incrementing the credit register by a value of the credits determined to be in the packet. 